The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex. The complex consists of an anterior disc and two posterior facet joints. The anterior discs of adjacent bones are cushioned by cartilage spacers referred to as intervertebral discs. The over 20 bones of the spinal column are anatomically categorized as one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine which comprises the top of the spine up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine are sacral bones, including the coccyx.
The spinal column of bones is highly complex in that it includes over 20 bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. Despite its complexity, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
Genetic or developmental irregularities, trauma, chronic stress, tumors and disease, however, can result in spinal pathologies which either limit this range of motion or threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior or lateral implants. Lateral and anterior assemblies are coupled to the anterior portion of the spine which is in the sequence of vertebral bodies. Posterior implants generally comprise pairs of rods (“bilateral spinal support rods”), which are aligned along the axis which the bones are to be disposed, and which are then attached to the spinal column by either hooks which couple to the lamina or attach to the transverse processes, or by screws which are inserted through pedicles.
Spinal fusion treatment is commonly used to treat spinal disc disease and/or spinal instability. The degeneration of spinal discs can create significant pain and discomfort for individuals suffering from this affliction. In many cases, this pain can be alleviated by immobilizing the vertebrae adjacent to the degenerated disc and encouraging bone growth across the immobilized area of the spine. Conventional spinal implants are designed to facilitate bone through-growth, or fusion resulting from growth of bone through holes or channels through the implants. Although effective, the bone through-growth process is slow, sometimes taking more than a year to complete. Through-growth can be further delayed if the implant area is not immobilized. Even micro-motion of the implant area can disturb and disrupt bone growth, leading to increased incidence of subsidence and pseudarthrosis.
Some conventional devices attempt to improve implant stabilization by encouraging bone on-growth—a comparatively rapid, planar growth of bone upon surfaces of an adjacent implant, or upon surfaces of adjacent bone. For example, on-growth may be encouraged by coating a titanium cage with a chemical such as hydroxyapatite to encourage new-grown bone to adhere to the implant surface. However, because titanium is radioopaque, titanium implants can interfere with diagnostic assessment of bone growth, whether coated with hydroxyapatite or not. For example, implants made primarily of radio-opaque titanium may obscure visualization of bone growth (e.g., through-growth) on x-rays. Titanium may likewise cause signal artifact with MRIs or CTs, making it difficult to determine if fusion has occurred.
In order to avoid the visualization problems of titanium implants, attempts have been made to mix hydroxyapatite with, or apply hydroxyapatite to, radiolucent polymer plastics (e.g., PEEK, HDPE, or other non-scattering biocompatible materials) to form a cage/implant. However, PEEK provides poorer fixation than titanium, and thus, PEEK implants often require supplemental fixation such as posterior pedicle screws and rod instrumentation.